Field of the Invention
The present invention relates to a magnetic-disk glass substrate and a magnetic disk.
Background Information
Nowadays, personal computers, digital versatile disc (DVD) recorders, and the like have a built-in hard disk drive (HDD) for data recording. In particular, a magnetic disk in which a magnetic layer is provided on a glass substrate is used in a hard disk drive that is used in a device premised on portability, such as a notebook-type personal computer, and magnetic recording information is recorded on or read from the magnetic layer with a magnetic head that flies slightly above the surface of the magnetic disk. At this time, the rotation rate of the magnetic disk is about 5400 rpm, for example. A glass substrate having a higher rigidity and a higher impact resistance than a metal substrate (aluminum substrate) or the like has is preferably used as a substrate of the magnetic disk. It should be noted that the magnetic-disk glass substrate generally has a thickness of 0.635 mm or 0.8 mm in a 2.5-inch magnetic disk, for example.
In recent years, there have been calls for a magnetic-disk for use in a stationary server device such as a network server device that is rotated at a higher rotation speed in order to further improve the access time and the transfer rate. In the case where a magnetic disk is rotated at a higher rotation speed than previously, a conventional magnetic-disk aluminum substrate does not have enough rigidity even if the thickness is increased, and therefore, fluttering occurs. Here, fluttering is a phenomenon in which a substrate vibrates (flutters) due to the rotation of the substrate. Therefore, JP 2008-226376A discloses a magnetic-disk glass substrate in which ΔS that is a difference between a maximum value Smax and minimum value Smin of the gap S between the inner circumference and the outer circumference of a projection image projected on a plane parallel with a main surface is set to less than 2 μm in order to reduce fluttering at a high rotation speed.